This invention relates generally to an improved high dielectric constant sheet material for use in electronic circuitry and electronic components. More particularly, this invention relates to a high dielectric constant sheet material which is drillable, platable and is well suited for a variety of uses including high current laminated bus bars, multilayer printed circuit boards, flexible circuits and decoupling capacitors.
It will be appreciated that there is an ever increasing need for a reliable, high dielectric material which may be used for a variety of applications in electronic circuitry design and manufacture. Presently, flexible high dielectric materials of this type are manufactured by mixing small particles (for example 1-3 microns) of a high dielectric constant material into a flexible polymeric matrix. Surprisingly, the resultant effective dielectric constant of the dielectric impregnated polymer is relatively low. For example, the dielectric constant of a Z5U BaTiO.sub.3 is in the range of 10,000 to 12,000. However, when such Barium Titanate is mixed with a flexible polymer such as polyimide, polyester, polyetherimide and like materials, the effective dielectric constant realizable is only on the order of 20 to 40 (depending on the loading ratio of the dielectric in the polymer).
An improved high dielectric sheet material is described in U.S. Pat. No. 4,908,258, which is assigned to the assignee hereof and incorporated herein by reference. The high dielectric sheet material of U.S. Pat. No. 4,908,258 comprises a monolayer of multilayer or single layer high dielectric (for example ceramic) chips or pellets of relatively small area and thickness which are arranged in a planar array. These high dielectric constant chips are spaced apart by a small distance. The spaces between the chips are then filled with a flexible polymer/adhesive to define a cohesive sheet with the polymer binding the array of high dielectric (for example ceramic) chips together. Next, the opposite planar surfaces of the array (including the polymer) are metallized (for example, electroless plated or metallized by vacuum deposition, sputtering, etc.) to define opposed metallized surfaces. The end result is a relatively flexible high capacitance dielectric film or sheet material which is drillable, platable, printable, etchable, laminable and reliable.
The high dielectric sheet material of U.S. Pat. No. 4,908,258 is suitable for many applications. For example, U.S. Pat. No. 4,853,827 (assigned to the assignee hereof and incorporated herein by reference) relates to a multilayer capacitor whose construction is based on the material of U.S. Pat. No. 4,908,258. Similarly, U.S. Pat. No. 5,051,542 (assigned to the assignee hereof and incorporated herein by reference) relates to a bus bar whose construction incorporates the material of U.S. Pat. No. 4,908,258. Finally, U.S. application Ser. No. 291,531 filed Dec. 29, 1988, now U.S. Pat. No. 5,065,285 issued Nov. 12, 1991 (assigned to the assignee hereof and incorporated herein by reference) also utilizes the sheet material of U.S. Pat. No. 4,908,258 in the construction of a multilayer printed wiring board.
While well suited for their intended purposes, the materials and devices of U.S. Pat. Nos. 4,908,258, 4,853,827, 5,051,542 and USSN 291,531 do suffer from certain drawbacks and deficiencies in terms of the relatively complicated and error prone process required during manufacturing. These prior art high dielectric sheet materials (and devices constructed which incorporates these materials) utilize complicated fixturing and polymer resin mold/casting tools to create a polymer matrix with exposed high dielectric chip surfaces. It will be appreciated that it is extremely difficult to make fixturing and polymer resin mold/casting tools to hold and orient very thin, fragile ceramic chips and thereafter fill the space between the chips without polymer resin flashing onto the external surfaces of the chips. Moreover, the relatively expensive and complicated tooling required to manufacture the prior art high dielectric sheet material significantly limit the maximum area of the sheet (e.g., 9 inch by 9 inch maximum).